Thermoplastic substrates, such as, for example, polycarbonates, are in general characterized by many advantageous properties, which include clarity, high ductility, high heat distortion point, and also dimensional stability. Many of these materials are transparent and are in some cases employed as a substitute for glass in commercial uses.
While thermoplastic resins have the advantageous properties described above, they often show too low a resistance to abrasion and chemical solvents for some uses. Furthermore, like many other organic polymeric materials, they are also sensitive to degradation by ultraviolet light. This leads to yellowing and erosion of the substrate surface.
For these reasons, thermoplastic substrates, such as, for example, polycarbonate components, are often provided with a protective coating. In this context, precisely those protection systems which form both a mechanical protection against abrasion and scratching and an excellent protection against weathering influences, i.e. rain, temperature and, in particular, ultraviolet radiation (UV), are suitable for external use.
The coating systems which are suitable for this and are used, for example, for polycarbonate sheets in the construction sector, for headlamp cover screens of polycarbonate or also in the field of polycarbonate automobile glazing can be roughly divided into three categories.
First, (a) heat curing coating systems based on a polysiloxane coating, which can be either single-layered or multi-layered (with a “merely adhesion-promoting primer layer” between the substrate and polysiloxane topcoat). These are described, inter alia, in U.S. Pat. Nos. 4,278,804, 4,373,061, 4,410,594, 5,041,313 and EP-A-1 087 001. There are mentioned by way of example here the commercially obtainable systems of Momentive Performance Materials Inc. Wilton, Conn. USA, such as PHC 587; PHC 587B; SHP 401 (primer)/AS 4000 (topcoat) or also SHP 401 (primer)/AS 4002 (topcoat) as well as KASI Flex® or Sun Flex®, both from KRD Coatings, Geesthacht, Germany, or Silvue® MP 100, SDC Coatings, Germany, or Sicralan® MRL from GFO, Schwäbisch Gmünd, Germany. A “merely adhesion-promoting primer layer” between the substrate and polysiloxane topcoat is a primer layer comprising an adhesion-promoting polymer and optionally one or more UV-absorbers in low concentration and/or with low layer thickness. For conventional, thick primer systems, these layer thicknesses of between about 0.3-1.5 μm are too low.
However, these are not yet adequately weather-resistant for some cases of use. To achieve a longer life of the components, improved systems have been developed here. A successful variant is the use of the adhesive primer necessary for the siloxane-based topcoat as a UV protection primer by adding a UV absorber to this and applying it in a higher layer thickness.
Second, (b) heat curing multi-layer systems having a UV protection primer and a topcoat based on a polysiloxane coating. Suitable systems are known e.g. from U.S. Pat. Nos. 5,391,795 and 5,679,820 and “Paint & Coating Industry; July 2001 page 64 to 76: The Next Generation in Weatherable Hardcoats for Polycarbonate” by George Medford/General Electric Silicones, LLC, Waterford, N.Y.; James Pickett/The General Electric Co., Corporate Research and Development, Schenectady, N.Y.; and Curt Reynolds/Lexamar Corp., Boyne City, Mich. A commercially obtainable system is the SHP470 (V protection primer)/AS4700 (topcoat) system from Momentive Performance Materials mentioned therein. This SHP470 is an adhesion-promoting UV protection primer based on polymethyl methacrylate, inter alia with 1-methoxy-2-propanol and diacetone alcohol as the solvent and dibenzoylresorcinol as the UV absorber. The AS4700 topcoat is a polysiloxane topcoat with a silylated UV absorber.
Further useful systems are those described in pending U.S. patent application Ser. No.: 12/254,387, the entire contents of which are incorporated herein by reference, comprising:
(a) 100,000 parts by weight of a primer composition comprising a binder material (a1), a primer solvent (a2) and a UV absorber (a3), wherein the primer composition is suitable as a adhesion-promoting agent between a thermoplastic substrate and a siloxane-based top coat;
(b) 0 to 900,000 parts by weight of a solvent, and
(c) 1 to 3000 parts by weight of a compound according to the general formula (I):
wherein X represents a moiety selected from the group consisting —OR6, —OCH2CH2OR6, —OCH2CH(OH)CH2OR6 and —OCH(R7)COOR8, wherein R6 represents a branched or unbranched C1-C13 alkyl, C2-C20 alkenyl, C6-C12 aryl or —O—C1-C18 alkyl, R7 represents H or a branched or unbranched C1-C8 alkyl, and R8 represents a C1-C12 alkyl, a C2-C12 alkenyl or a C5-C6 cycloalkyl, and wherein the composition has a viscosity of 40 s to 140 s measured in accordance with DIN EN ISO 2431 at 23° C. and with a 2 mm cup. Both layers, i.e. primer and top coating together assume the function here of the UV protection finish.
Third, (c) UV curing coating systems, e.g. based on acrylate, urethane acrylate or acrylosilane, which optionally contain fillers for improving the scratch resistance, can likewise form an adequate weather protection because of their relatively wide application layer thickness window. Such systems are known and are described, inter alia, in U.S. Pat. No. 3,707,397 or DE 69 71 7959, U.S. Pat. Nos. 5,990,188, 5,817,715 and 5,712,325. The commercially obtainable UVHC 3000, UVHC 3000K and UVHC 3000S systems from Momentive Performance Materials and UVT 200 and UVT 610 from Redspot are examples of such a suitable coating.
The weather-resistant scratch protection layers (a) and the highly weather-resistant scratch protection layers (b) and (c) are also used on other thermoplastic substrates, such as e.g. polycarbonate blends.
In many planar uses with one side exposed to weather, not only is a protective layer required on this outside, but the inside which is exposed less to weather must likewise be protected against scratching, the action of chemicals and, to a reduced extent compared with the outside, also against UV attack. In these cases the inside of the component is likewise co-coated directly, i.e. with the same coating system.
DE 3044450 A1 relates to an impact- or shock-resistant laminate having a number of layers which absorb impact or shock chosen from polycarbonates, glasses and solid resin materials, including a backwards polycarbonate layer with respect to the direction of impact or shock, wherein this polycarbonate layer has, on the side opposite to the direction of impact, a scratch-resistant coating which is harder and more brittle than the underlying polycarbonate layer, and an intermediate layer, which is less fragile than the scratch-resistant coating, arranged between the backwards polycarbonate layer and the scratch-resistant coating, the layers being bonded to one another with a compatible adhesive.
DE 10 2004014023A1 relates to a composite screen of plastic having an inside and an outside for vehicle glazing, characterized in that that the inside screen and the outside screen are joined by a layer of thermoplastic polyurethane.
While the heat curing coating systems (a) are often not yet sufficiently weather-resistant, a dramatic drop in the toughnesses of the overall component, especially at low temperatures, is to be observed in the case of components provided similarly on both sides with scratch protection layers (b) and (c) which are highly weather-resistant. Low temperatures here means below ≦0° C., in particular <−10° C.≈0 to about −30° C. In puncture tests on components of polycarbonate and blends thereof with highly weather-resistant scratch-resistant coatings, a loss in low temperature toughness under dynamic load was found here after the coating on both sides.